This invention relates to distance measurement devices, and more particularly to distance measurement devices mounted on automotive vehicles for measuring the inter-vehicular distances (i.e., the distances to preceding vehicles), so as to control the speeds of the vehicles and the inter-vehicular distances. This invention also relates to driving control devices for automotive vehicles for maintaining the distance to the preceding vehicle by controlling the vehicle velocity on the basis of the measurement of the inter-vehicular distance.
FIG. 6 is a block diagram showing the structure of a trigonometric or triangulation type distance measurement device, which is disclosed, for example, in Japanese Patent Publication (Kokoku) No. 63-46363. FIG. 7 is a block diagram showing the structure of another triangulation type distance measurement device, which is disclosed, for example, in Japanese Patent Publication (Kokoku) No. 62-33522. FIG. 8 is a block diagram showing the structure of still another triangulation type distance measurement device, which is disclosed, for example, in Japanese Patent Publication (Kokoku) No. 63-44169.
The principle of the distance measurement by means of the triangulation type distance measurement device is described by reference to FIG. 6. The distance measurement device of FIG. 6 includes: first and second optical lens systems 2 and 12 separated by a lateral distance L; image sensors 3 and 13; analog-to-digital converters 4 and 14; RAMs (random access memories) 5 and 15; an image processor circuit 20; and a computer (CPU) 21.
The method of operation of the distance measurement device of FIG. 6 is as follows. The object 1 the distance to which is to be measured is at a distance R from the lenses 2 and 12 separated by a lateral distance L. Thus, the images of the object 1 formed on the image sensors 3 and 13, respectively, via the optical lens systems 2 and 12, are displaced by lateral distances a and b, respectively, from the centers of the image sensors 3 and 13 (i.e., the intersections with the optical axes of the lens systems 2 and 12). The images upon the sensors 3 and 13 are converted into corresponding analog electrical signals, and further into corresponding digital signals by means of the analog-to-digital converters 4 and 4. The digital image signals output from the analog-to-digital converters 4 and 14 are stored in the RAMs 5 and 15, respectively.
The computer (CPU) 21 reads out the digital image signals successively from the RAMs 5 and 15, and feeds them to the image processor circuit 20. Upon receiving the digital image signals, the image processor circuit 20 shifts the image of the RAM 5 (obtained via the optical lens system 2), for example, successively by a pixel relative to the image of the RAM (obtained via the optical lens system 12), and calculates the correlation (i.e., the degree of agreement) between the two images stored in the RAMs 5 and 15.
Namely, if the amount of the lateral shift measured in units of the lateral pitch of the pixels of the digital images is denoted by X, the image processor circuit 20 shifts the image of the RAM 5, for example, relative to the image of the RAM 15 by an amount X, and calculates the correlation C(X) of the two images at the amount of shift X. The correlation may be defined, for example, by C(X)=-S(X), where S(X)=.SIGMA..vertline.p-q.vertline., the summands .vertline.p-q.vertline. being the absolute values of the differences between the levels p and q of the corresponding pixels of the images in the RAMs 5 and 15, respectively. The amount of the shift X is successively incremented or decremented by a pixel pitch, to determine the value X at which the correlation C(X) takes the maximum value. The maximizing value X is the relative shift a +b of the images formed upon the image sensors 3 and 13.
Thus, the distance R to the object 1 is obtained by the following equation: EQU R=f.multidot.L/{(a+b).multidot.P} (1)
where f represents the focal distance of the lens systems 2 and 3, and P represents the lateral pitch of the pixels (i.e., the lateral separation between the centers of adjacent pixels) of the image sensors 3 and 13.
The distance measurement device of FIG. 7 includes, in addition to an optical lens system 2 and an image sensor 3, a light-emitting element 11 whose optical axis runs parallel to the optical axis of the optical lens system 2, at a lateral separation L. When the distance to an object 1 is measured, the light-emitting element 11 is driven by a driver circuit 16 in response to a control signal received from the computer (CPU) 21. A beam of light emitted from the light-emitting element 11 is reflected from the object 1 to the optical lens system 2. Thus, an image of the object 1 or a bright spot is formed upon the image sensor 3 at a lateral distance a from the center of the image sensor 3 (the intersection with the optical axis of the optical lens system 2). The signal from the image sensor 3 is fed to the signal processor circuit 20A, which determines the shift a of the spot formed upon the image sensor 3 with respect to the reference position (the intersection of the optical axis of the optical lens system 2 with the image sensor 3).
Thus, the distance R to the object 1 is obtained by the following equation: EQU R=f.multidot.L(a.multidot.P) (2)
where f represents the focal distance of the optical lens system 2, and P represents the lateral pitch of pixels of the image sensor 3.
The distance measurement device of FIG. 8 is similar to that of FIG. 7, but includes an analog image sensor 3A instead of a digital image sensor 3 of FIG. 7. The signal from the analog image sensor 3A is fed to the signal processor circuit 20B. If the shift of the image formed upon the analog image sensor 3A as measured in meters, is represented by d, the distance R to the object 1 is obtained by the following equation: EQU R=f.multidot.L/d (3)
where f represents the focal distance of the optical lens system 2.
The distance measurement devices of FIGS. 6 through 8 may be mounted on automotive vehicles to determine the inter-vehicle distances. Under such circumstances, the object 1 is the preceding vehicle which is running just ahead. It goes without saying that the distance measurement devices can be used alone to determine distances to any kind of object 1.
The distance measurement devices of FIGS. 6 through 8 are of the trigonometric or triangulation type. Japanese Laid-Open Patent ( Kokai ) No. 48-54631 and Japanese Patent Publication (Kokoku) No. 62-38760, on the other hand, disclose a vehicular distance measurement device which is not of the triangulation type, including: means for measuring the distance to a vehicle running ahead; means for determining a safe inter-vehicular distance; and means for controlling the velocity of the vehicle to keep the safe inter-vehicular distance. The distance measurement device uses a radar which emits a light or electromagnetic beam and, upon receiving the beam reflected from the object, determines the distance thereto from the length of time required by the beam to travel forward and backward. Provided that the beam hits on the object (the vehicle running ahead) correctly, the error of the measurement by the radar arises from the error the measurement the time length. The error is substantially independent of the magnitude of the distance to be measured.
In the case of the triangulation type distance measurement device, however, the distance R to the object is inversely proportional to the shift of the image. Thus, the error of distance measurement increases (and hence the resolution of the measurement decreases) as the distance to the object increases. The error can be generally regarded as random and the measurement fluctuates around the true distance.
The inter-vehicular distance measurement device using radar, on the other hand, has a problem from another cause. Namely, the vehicle on which the radar is mounted may shake or sway due to the road condition. The light or electromagnetic beam emitted from the radar mounted upon the running vehicle thus swings vertically and horizontally. The beam may mis-hit the preceding vehicle the distance to which is to be measured. The beam hits upon the road surface or a building at the road side, and is reflected therefrom to the radar. The measurement thus fluctuates. The fluctuation generally becomes greater as the distance to be measured increases. Further, when the distance to be measured is great, a small error in the direction of the beam may cause a mis-hit. The problem thus becomes more serious as the distance to be measured increases.
If the vehicle velocity is controlled on the basis of the measurement which fluctuates as discussed above, the vehicle velocity tend to follow the fluctuation of the measurement. The driving comfort is thus largely impaired. Reducing the control gain to improve the driving comfort causes another problem, since it also reduces the response speed. The driving safety may thus be placed in peril if a succeeding vehicle suddenly comes forward and forces its way between the driver's and the preceding vehicles.